Hornwort

Hornworts are a group of non-vascular Embryophytes (land plants) constituting the division Anthocerotophyta (ˌænθoʊˌsɛrəˈtɒfətə,_-təˈfaɪtə). The common name refers to the elongated horn-like structure, which is the sporophyte. As in mosses and liverworts, hornworts have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information; the flattened, green plant body of a hornwort is the gametophyte stage of the plant. Hornworts may be found worldwide, though they tend to grow only in places that are damp or humid. Some species grow in large numbers as tiny weeds in the soil of gardens and cultivated fields. Large tropical and sub-tropical species of Dendroceros may be found growing on the bark of trees. The total number of species is still uncertain. While there are more than 300 published species names, the actual number could be as low as 100-150 species. Like all bryophytes, the dominant life phase of a hornwort is the haploid gametophyte. This stage usually grows as a thin rosette or ribbon-like thallus between one and five centimeters in diameter. Hornworts have lost two plastid division-associated genes, ARC3 and FtsZ2, and have just a single chloroplast per cell (monoplastidy), with the exception of the genus Megaceros and some species in the genera Nothoceros and Anthoceros, which have more than one chloroplast per cell (polyplastidy). In the polyplastidic species, and also some of the monoplastidic species, a cellular structure called a pyrenoid is absent. The pyrenoid, which is both a food storing organ and enables a more efficient photosynthesis, have evolved independently five to six times in hornworts and is present in half of the roughly 200 species. It is formed by the fusion of the chloroplast with other organelles and is comprised predominantly of RuBisCO, the key enzyme in carbon fixation. By using inorganic carbon transporters and carbonic anhydrases, up to a 50-fold increase in CO2 levels can be achieved.